Sébastien Alzuaga

Surface acoustic wave devices as passive buried sensors

Surface acoustic wave (SAW) devices are currently used as passive remote-controlled sensors for measuring various physical quantities through a wireless link. Among the two main classes of designs—resonator and delay line—the former has the advantage of providing narrow-band spectrum informations and hence appears compatible with an interrogation strategy complying with Industry-Scientific-Medical regulations in radio-frequency (rf) bands centered around 434, 866, or 915 MHz. Delay-line based sensors require larger bandwidths as they consists of a few interdigitated electrodes excited by short rf pulses with large instantaneous energy and short response delays but is compatible with existing equipment such as ground penetrating radar (GPR). We here demonstrate the measurement of temperature using the two configurations, particularly for long term monitoring using sensors buried in soil. Although we have demonstrated long term stability and robustness of packaged resonators and signal to noise ratio compat...

Detection and High-Precision Positioning of Liquid Droplets Using SAW Systems

The capability to accurately handle liquids in small volumes is a key point for the development of lab-on-chip devices. In this paper, we investigate an application of surface acoustic waves (SAW) for positioning micro-droplets. A SAW device based on a 2 times 2 matrix of inter-digital transducers (IDTs) has been fabricated on a (YXl)/128deg LiNbO3 substrate, which implies displacement and detection in two dimensions of droplets atop a flat surface. Each IDT operates at a given frequency, allowing for an easy addressing of the active channel. Furthermore, very low cross-talk effects were observed as no frequency mixing arose in our device. Continuous as well as pulsed excitations of the IDTs have been studied, yielding, respectively, continuous and step-by-step droplet displacement modes. In addition, we also have used these two excitation types to control the velocity and the position of the droplets. We also have developed a theoretical analysis of the detection mode, which has been validated by experimental assessment.

High overtone Bulk Acoustic Resonators built on single crystal stacks for sensors applications

This paper describes the principles of single crystal LiNb03/Quartz high overtone bulk resonators and their application to the development of compact sensors dedicated to temperature and stress measurements. Design approaches are presented and first experimental results demonstrating the operation of such sensors are reported.

Droplet ejector using surface acoustic waves

The present study aims to investigate droplet ejection using Surface Acoustic Waves (SAW). The interests in droplet ejection microsystems have dramatically grown in recent years due to inkjet printhead demand (Drop-On-Demand). Among several actuating methods (thermal, piezoelectric, etc.), the thermally driven inkjet printhead is the most successful (low cost, easy fabrication and high printing quality). Almost all of the current ink jet printers eject ink droplets through nozzles, with a direction of ejection always perpendicular to the nozzle surface. Surface acoustic waves devices are widely used for frequency filtering and are mainly devoted to cellular phones and telecommunication handset. Nowadays, recently published works have demonstrated the interest of SAW for guiding and positioning small liquid droplet atop a flat surface. This paper describes a new way to eject droplets (water, ink, etc...) using SAW. The surface acoustic waves devices used to eject droplets are carried out on lithium niobate substrates (LiNbO3 cut Y+128°, X propagation). The Rayleigh waves are excited using classical inter-digital transducers (IDT). The design of IDT has been simulated with finite element analysis and boundary element methods. The computations on the design of the IDT have been compared with measurements. The vibration amplitude of the wave necessary to droplet ejection is measured using a heterodyne laser probe. The range of the droplets volume ejected is between 100nl and 1μl. The influence of the supply voltage on the ejection is described.

Tunable and high quality factor SrTiO3 surface acoustic wave resonator

We fabricated a tunable surface acoustic wave resonator in the 2 GHz-frequency range by depositing and patterning 2 μm-wide pitch inter-digitated Al electrodes on SrTiO3 (STO) paraelectric substrate. We took advantage of the electrostrictive behavior of STO, whose properties are nonlinear with respect to the applied electric field, to induce tunability of the resonance frequency. The obtained frequency tunability reaches 0.7% at 0.5 MV/cm. Besides, the main advantage of this device is its high acoustic quality factor Q reaching 2450 at 2 GHz, thanks to the single-crystal nature of STO. This is one order of magnitude larger than the typical quality factor of its tunable bulk acoustic wave resonators counterparts.

Novel narrowband acoustic sensors for sub-GHz wireless measurements

High-overtone Bulk Acoustic Resonator is an acoustic transducer based on an excitation of a bulk acoustic wave by a thin piezoelectric film bonded to a thick low acoustic loss substrate. This combination of materials aims at providing on the one hand a high frequency transducer as defined by the thickness of the thin piezoelectric layer, and on the other hand the robustness of a thick substrate while keeping the acoustic properties of single crystal piezoelectric materials. More specifically, this architecture provides high quality factors using bulk acoustic wave at frequencies only accessible to surface acoustic wave (SAW) devices with interdigitated transducer generation. The multimode spectrum is well suited for an openloop, wireless interrogation strategy in which the frequency of the incoming electromagnetic wave defines the operating point. We here demonstrate the use of a frequency sweep RADAR-like network analyzer for probing through a wireless link HBARs with different temperature coefficients in order to perform temperature measurements insensitive to other correlated noise sources (capacitive frequency pulling, electrode aging, stress).

Piezoelectric radiofrequency transducers as passive buried sensors

We demonstrate that single-piezoelectric substrate-based acoustic transducers act as ideal sensors for probing with various RADAR strategies. Because these sensors are intrinsically passive devices working in the radiofrequency range, they exhibit improved interrogation range and robustness with respect to silicon-based radio frequency identification tags. Both wideband (acoustic delay lines) and narrowband (acoustic resonators) transducers are shown to be compatible with pulse-mode and frequency-modulated continuous-wave RADAR strategies, respectively. We particularly focus on the ground-penetrating RADAR (GPR) application in which the lack of local energy source makes these sensors suitable candidates for buried applications in roads, building or civil engineering monitoring. A novel acoustic sensor concept – high-overtone bulk acoustic resonator – is especially suited as sensor interrogated by a wide range of antenna set, as demonstrated with GPR units working in the 100 and 200 MHz range.

Electron-beam processed SAW devices for sensor applications

In this paper, electron-beam (e-beam) lithography for processing of surface acoustic wave devices is investigated, and its suitability for large-scale processing discussed. Electron-beam lithography is used for exposure of surface acoustic wave (SAW) resonator patterns on polymethyl methacrylate (PMMA) coated piezoelectric substrates. Electron-beam lithography can be used for high frequency SAW designs, due to a minimal finger width of 100 nm to 400 nm. Such SAW devices can be used for high-frequency sensor applications. This contribution will consider processing, on-wafer characterization, and characterization of sensor effects in instrumentation applications.

Acoustic Transducers as Passive Cooperative Targets for Wireless Sensing of the Sub-Surface World: Challenges of Probing with Ground Penetrating RADAR

Passive wireless transducers are used as sensors, probed by a RADAR system. A simple way to separate the returning signal from the clutter is to delay the response, so that the clutter decays before the echoes are received. This can be achieved by introducing a fixed delay in the sensor design. Acoustic wave transducers are ideally suited as cooperative targets for passive, wireless sensing. The incoming electromagnetic pulse is converted into an acoustic wave, propagated on the sensor substrate surface, and reflected as an electromagnetic echo. According to a known law, the acoustic wave propagation velocity depends on the physical quantity under investigation, which is then measured as an echo delay. Both conversions between electromagnetic and acoustic waves are based on the piezoelectric property of the substrate of which the sensor is made. Investigating underground sensing, we address the problems of using GPR (Ground-Penetrating RADAR) for probing cooperative targets. The GPR is a good candidate for this application because it provides an electromagnetic source and receiver, as well as echo recording tools. Instead of designing dedicated electronics, we choose a commercially available, reliable and rugged instrument. The measurement range depends on parameters like antenna radiation pattern, radio spectrum matching between GPR and the target, antenna-sensor impedance matching and the transfer function of the target. We demonstrate measurements at depths ranging from centimeters to circa 1 m in a sandbox. In our application, clutter rejection requires delays between the emitted pulse and echoes to be longer than in the regular use of the GPR for geophysical measurements. This delay, and the accuracy needed for sensing, challenge the GPR internal time base. In the GPR units we used, the drift turns out to be incompatible with the targeted application. The available documentation of other models and brands suggests that this is a rather general limitation. We solved the problem by replacing the analog ramp generator defining the time base with a fully digital solution, whose time accuracy and stability relies on a quartz oscillator. The resulting stability is acceptable for sub-surface cooperative sensor measurement.

Stress-sensitivity of wafer-level packaged SAW delay lines

This paper presents the investigation of the influence of wafer-level packaging (WLP) on the stress-sensitivity of 100 and 200 MHz delay lines aimed to wireless sensing of stresses. The devices were fabricated on YXl/128° cut of lithium niobate. The investigated WLP achieves the assembly of two wafers by a 50μm-thick layer of SU-8 photoresist. The delay line is micro-machined on top of the first wafer while the second wafer realizes the function of protective cap in a way that should not be detrimental to the stress-sensitivity of the device. The paper gives a comparison between the theoretical and experimental phase sensitivity of both packaged and raw devices submitted to a three-points bending test.